The present invention generally relates to potential estimation apparatuses, and more particularly to a potential estimation apparatus which is suited for estimating a potential of an electro-photographic photosensitive body of an image forming apparatus that carries out an electrophotography process. In other words, the potential estimation apparatus is suited for use in a copying machine, a printer, a facsimile machine and the like which carry out image formation such as copying and printing by the electrophotography process.
Conventionally, various methods have been proposed to control the latent image in the electro-photography process. For example, there is a first method which measures the surface potential of a photosensitive drum using a surface electrometer, and looks up values for control input such as the charger voltage, the charging grid voltage and the exposure lamp voltage from a table using each measured value. This table prestores values (voltages in this case) for the control input which are obtained in advance for various measured values, and the latent image is controlled based on the values read from the table. On the other hand, there is a second method which feeds back the state of the image forming apparatus via sensors or the like while changing the control input, so as to find optimum control input by a PID control, for example.
However, according to the first method which looks up the table, there was a problem in that it is difficult to correctly grasp the characteristics of the photosensitive drum. In addition, according to the second method which uses the feedback control, the feedback loop must be repeated a plurality of times, that is, the charging and exposing processes are repeated, until an ideal controlled state is reached. For this reason, it takes time until the ideal controlled state is reached according to this second method, and there were problems in that the performance of the image forming apparatus itself deteriorates. In other words, the image forming speed per unit time deteriorates according to the second method, and it takes a long time until a first copy or print is formed by the image formation.
On the other hand, there is a proposed method which carries out a control to constantly form an image of a high quality by correcting the deterioration of the sensitivity based on the number of copies or prints made and the total rotational time of the photosensitive drum. However, this proposed method had a problem in that it is impossible to correct the deterioration of the potential characteristic which occurs on a short term basis due to the repetitive charging, exposure and discharging of the photosensitive drum.